What is it exactly about these flares of infrared light from Sgr A* that confirms it is a supermassive black hole?

Certainly there is plenty of evidence, the orbits of stars nearby for one. But this use of the word confirm seems strong and frequent enough to suggest the deal has been done, the ink is dry, and it's a black hole without question and that until now that wasn't so.

I've shamelessly absconded the following images from this answer to the question What is the evidence for a supermassive black hole at the center of Milky Way?, which may need a new answer!

Note, the size of these stellar orbits are compared to those of Sedna, Eris, Pluto and Neptune in the bottom right corner (same scale)!

$\begingroup$@CarlWitthoft While I did change my profile picture during Halloween to "scare" people, these quotes are the friendly variety and not meant to scare anybody. I used friendly quotes intentionally, and a careful read through the answer, and how the quoted word got into the popular press will show why I did so.$\endgroup$
– uhohNov 2 '18 at 15:55

$\begingroup$@CarlWitthoft I have to admit though I'd never heard of that term before; I learn something new every day in Stack Exchange!$\endgroup$
– uhohNov 2 '18 at 16:04

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$\begingroup$Do not remove the quotes around the word "confirm" in the title. It is correct usage. (They are putative quotes here, not "scare" quotes.)$\endgroup$
– FattieNov 3 '18 at 3:20

1 Answer
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We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the central object is $M\approx4\times10^6M_{\odot}$; this alone is fairly good evidence for a supermassive black hole, and we can constrain the size of the object with the measurements (Ghez et al. 2008). Models of the orbits of other stars have further improved these results.

The recently-published paper (Abuter et al. 2018) uses a similar technique. The motion of the flares is well-described by the rotation of a "hot spot" of gas in the inner reaches of the accretion disk, with the flares arising from magnetic recombination or some similar event. In particular, it lies near the innermost stable prograde circular orbit (ISCO) - just outside it, actually (the orbital radius can be found because we know the mass of the object and the period of the orbit, with the best-fit model of the latter being $40\pm 8$ minutes). This both constrains the object's size and even qualitatively provides support for the black hole model, as we might expect to see such events around a supermassive black hole.

Looking at those articles, I think the use of the word "confirm" is inaccurate. As far as I can tell, the term is only used by the project leader, Reinhard Genzel, in that ESO statement - and it's not a claim repeated in the paper. The team describes their results as "strong support" for the supermassive black hole model, and say that their findings are "consistent with" that theory. As scientists should be, they're cautious. The results don't definitely confirm that Sagittarius A* corresponds to a supermassive black hole; they're simply additional (excellent) evidence for it.

$\begingroup$To simplify the core observation. (1) we can see stars moving around the central mass, which tells us how much it masses. (2) we can now detect a source of IR orbiting it every 40 minutes -- which implies that it must also be pretty small. Anything which is massive and small enough must be a black hole.$\endgroup$
– Steve LintonNov 2 '18 at 7:51

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$\begingroup$@uhoh, sure. I was trying to extract the minimal answer to the question asked, in as simple a way as possible. The answer certainly offers more.$\endgroup$
– Steve LintonNov 2 '18 at 11:40